Use goma only if --gomadir is set.
Use goma only if --gomadir is set.

BUG=516552

Review URL: https://codereview.chromium.org/1256313004

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client